1. Field of the Invention
The invention relates to a method for obtaining a high level of adhesion between rubber and metal and retaining such adhesion after thermal aging. More particularly, the invention relates to novel rubber compositions having a high level of adhesion to steel or steel plated with copper, zinc, copper alloys, zinc alloys, or copper and zinc alloys, and having improved thermal aging resistance.
2. Description of the Prior Art
It is essential during the manufacture of all rubber articles in which the rubber is reinforced with metal to provide a firm bond between the rubber composition and the metal. When reinforcing rubber articles such as tires, transmission belts, conveyor belts, hose and the like with metal wires, cords or plates, a high adhesion must be obtained between the rubber composition and the metal which must be retained after thermal aging to ensure the extended life and good performance of the rubber article.
Prior to the present invention, organic cobalt salts, such as cobalt salts of organic carboxylic acids, have been added to unsaturated rubbers, such as natural rubber, polyisoprene, polybutadiene and styrene-butadiene rubber, to provide adhesion between metal and the rubber composition. U.S. Pat. No. 3,514,370 discloses compounding lowly unsaturated rubbers with a cobalt salt of an organic carboxylic acid, such as cobalt naphthenate, cobalt oleate or cobalt stearate, to provide high adhesion to the metal surface. It has been found, however, that such rubber compositions lose adhesion upon thermal aging.
It has also been known that carboxylated divalent metal borates or aluminates such as those disclosed in U.S. Pat. No. 3,296,242 may be added to polymeric compositions as a stabilizer, or added to surface coatings as a drier. It has been found that when cobalt boron acyloxy compounds such as a carboxylated cobalt borate made in accordance with this patented process are compounded with rubber the adhesion between the rubber composition and a metal surface is greatly increased. These compositions likewise have a tendency to lose certain physical properties including adhesion as thermal aging proceeds.
Previously, metal oxides such as magnesium oxide and zinc oxide have been added to polymeric compositions to improve the retention of physical properties after thermal aging. Typical compositions are disclosed in U.S. Pat. Nos. 3,367,903 and 3,649,587. In addition, magnesium oxide has been added to rubber compounded with cobalt carboxylates such as those mentioned above to adhere the rubber composition to a metal surface. In U.S. Pat. No. 3,903,026, the rubber is compounded with a cobalt carboxylate and magnesium oxide to adhere the rubber composition to zinc or zinc alloy plated steel and to retain this adhesion after thermal aging. It was found, however, that these rubber compositions exhibit inferior original and thermally aged adhesions when, as is common, the metal surface varies in such characteristics as plating thickness, including bare spots, and alloy composition.